1. Field of the Invention
The present invention relates to a multi-layer optical recording medium and a method for evaluating a recording system including the same.
2. Description of the Related Art
Widely used optical recording media for recording digital data include compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs (BDs). To increase the recording capacity of these optical recording media, a multi-layer optical recording medium having a multi-layer structure in which a plurality of recording layers are stacked has been proposed.
As this multi-layer structure, a double layer structure has already been put into practical use, and further four to eight layer structures are now under study (described in Kenichi Nishiuchi et al., The Review of Laser Engineering, vol. 32, No. 1, pp 33-37 (2004)). In either case, a structure is employed in which recording layers are stacked with a transparent spacer layer (referred to below as a spacer layer) interposed between each layer and the next layer.
In such multi-layer optical recording media, when reproducing information in a certain recording layer, reflections from other recording layers inevitably occur (interlayer crosstalk). If the amounts of the reflections or a distance between recording layers vary due to some reason, there is a problem that the variation (referred to below as a crosstalk variation) is overlapped with a reproduced signal as a noise.
To reduce an effect of such interlayer crosstalk, as described in, for example, Japanese Patent Laid-Open Publication No. 2004-213720, a multi-layer optical recording medium is proposed in which the distances between recording layers are made different from each other. Another method, although not publicly known, has also been proposed in which the size and shape of the light receiving surface of a photo sensor are optimized and a reproduced signal is passed through a high-frequency filter, whereby a noise caused by the variation of a crosstalk is reduced.
Among various effects of the interlayer crosstalk in a multi-layer optical recording medium, a critical problem is the occurrence of a signal intensity variation.
Although in general optical recording media the variation of the reflectivity of a laser beam focused on a recording layer is detected as a signal, the variation of the reflectivity or signal intensity (signal intensity variation) is inevitably present even if data has not yet been recorded. The signal intensity variation is caused by, for example, the shape errors of grooves and pits provided on an optical recording medium, the variation of the thickness of a thin film forming a recording layer, or the unevenness of a surface or an interface.
The study by the inventors has found that the above signal intensity variation, which becomes pronounced in multi-layer optical recording media, is significantly increased not only simply by an increase in the number of recording layers, but also by a specific component of the crosstalk light beams (referred to below as a confocal crosstalk light beam) which is nearly focused on a photo sensor together with a signal light beam (this finding is not publicly known).
Accordingly, it becomes necessary to evaluate multi-layer optical recording media in consideration of such a signal intensity variation.
A conventional method of evaluating an optical recording medium is that the signal intensity variations are measured over the optical recording medium from inner to outer circumferences to quantify each circumference by using a degree of modulation as an indicator.
According to this method, when reproducing a certain region on an optical recording medium, the maximum and minimum values of signal intensity are measured to calculate the value of a degree of modulation given by the equation (maximum value−minimum value)/(maximum value); if this degree of modulation is, for example, 15 percent or less, good recording and reproduction are regarded to be possible.
When reproducing a multi-layer optical recording medium especially having three or more recording layers, however, since both the signal light beam and confocal crosstalk light beam are highly coherent in time and space, they easily interfere with each other. Therefore, even if the optical path length difference between the two increases or decreases by an amount equal to about a wavelength, the amount of received light increases or decreases due to the interference. In theory, it is possible to control the increase and decrease of this optical path length difference by uniforming the thicknesses of the spacer layers between recording layers, but in practice, it is impossible to uniform the thicknesses of the spacer layers with an accuracy of less than the wavelength of a reproducing laser beam (250 to 420 nm when the refractive index is assumed to be 1.56).
The degree of modulation of such inevitable signal intensity variation is given by the following equation (1).
                    (                  equation          ⁢                                          ⁢          1                )                                                            Mod        =                              4            ⁢                          a                                            1            +            α            +                          2              ⁢                              α                                                                        (        1        )            
In this equation, the symbol α denotes the received light intensity ratio of a crosstalk light beam with respect to a signal light beam, and the above degree of modulation occurs by an ideal interference.
This received light intensity ratio α is the light amount ratio between a crosstalk light beam and a signal light beam, wherein the amount of each light beam is determined by designs of the optical system and multi-layer optical recording medium and is obtained by actual measurements or from calculations by optical simulation. This received light intensity ratio α will be further described in detail below.
In the reproducing process of a multi-layer optical recording medium, a reproducing light beam is irradiated so as to focus on a recording layer (reproducing layer) to be reproduced. It is assumed here that: a light beam that is reflected only on the reproducing layer and emitted again to the outside of the multi-layer optical recording medium is referred to as a signal light beam; and light beams that are reflected on the recording layers other than the reproducing layer and a light beam that experiences multiple reflections of three times or more are referred to together as crosstalk light beams. The optical system is designed so that the signal light beam focuses again on a photo sensor after having passed through an objective lens. This implies that the reproducing layer and the photo sensor are designed so as to correspond to the object surface and the image surface of the objective lens, respectively. Since the light receiving area of the photo sensor is designed so as to be larger than the diffraction-limited beam diameter in the return path of the signal light beam, the signal light beam irradiated to the photo sensor is mostly detected on the light receiving surface. In the meantime, the crosstalk light beams take various light paths and are irradiated to the photo sensor, among which the amounts depending on their light paths are detected on the light receiving surface (the larger the amount of light not focusing on the light receiving surface, the smaller the amount of light detected). Therefore, by paying attention to the crosstalk light beam having the maximum amount of light detected on the light receiving surface, the received light intensity ratio of the amount of light detected on this light receiving surface with respect to a signal light beam is denoted by α.
Even if a received light intensity ratio of 1% is assumed, the degree of modulation Mod obtained from the equation (1) reaches as much as 33%. Although the signal intensity variation is actually reduced, for example, by effects of the aberrations occurring in an optical system or an optical recording medium, the degree of modulation is likely to become approximately 20 to 50% of the value given by the equation (1).
Although a method of reducing the interlayer crosstalk has been proposed in the patent document as described above, a method of accurately measuring an effect of the interlayer crosstalk has not been proposed.
Accordingly, there has been neither a method for evaluating a multi-layer optical recording medium nor a method for evaluating a performance of the system to reduce the signal intensity variation in a recording system including a multi-layer optical recording medium.